May. 10, 2013

According to one account, a small group of physicists at the University of Chicago were having lunch one day in 1950 with physicist Enrico Fermi, joking about the many newspaper articles reporting visits from UFOs. In one story a group of neighborhood kids had apparently stolen garbage can lids and tossed them like Frisbees in front of people’s windows. Neighbors thought the whizzing disks were otherworldly visitors.

Fermi, known to colleagues as “The Pope” because he seemed infallible, sat quietly for a few seconds while the laughter subsided and then asked, “Where is everybody?” He meant extraterrestrials.

Given the vastness of the cosmos, it seems incredible that Earthlings could be the first technological society. Assuming that intelligent life on other planets is common, Fermi supposed that the time any ambitious society would need to colonize the galaxy was a mere tens of millions of years—a small fraction of the much older Milky Way’s age. So, colonization should have happened by now. But we don’t see any evidence, such as feats of astroengineering, for example. “This struck Fermi as an interesting conundrum,” writes Seth Shostak, a senior astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute, in an email. The quandary became known as Fermi’s Paradox.

In 1961, astronomer Frank Drake, who conducted the first observational experiment for extraterrestrial life, worked out an equation to estimate the likely number of technological civilizations in the Milky Way. He based his thought experiment on factors such as the number of stars with planetary systems, as well as pure conjecture, and has concluded that there could be 10,000 detectable civilizations currently sharing our galaxy.

Yet, “There is no reason to restrict [the potential for extraterrestrial life] to one galaxy, says Chris Impey, a distinguished professor of astronomy at the University of Arizona. “If [extraterrestrials] were long-lived and advanced enough, they could travel between galaxies.”

Because of instruments like the Hubble telescope and the Kepler spacecraft, we now know more than ever about the universe’s age and the types of planets out there. It's possible that the number of planets in the visible universe that could support Earth-like life is on the order of 100 billion billion. In fact, just last month [April 2013], the Kepler mission discovered three more planets living in the “habitable zone” of their star—that is, the range of distance where an orbiting planet’s surface temperature might be suitable for liquid water (UPDATE: NASA has reported more planets in that zone since this article was originally reported). A person would have to be pretty pessimistic to be unable to imagine that on some fraction of such planets “interesting” life evolved, says Impey, such as civilizations capable of interstellar flight.

But the decades-old SETI experiment, which listens for unnatural radio signals from outer space—the kinds of transmissions an alien civilization may have sent—has heard nothing.

Ther are lots of possibilities to explain the silence.

Some of the quiet could have to do with time, says Impey. We’ve been listening to space for only 50 years and sending out radio and television signals for fewer than 100—a blink of an eye compared with the billions of years that the universe has been around. Civilizations may have come and gone thousands of years ago, without us knowing they ever existed.

And maybe technologically advanced alien civilizations haven’t communicated with us because they don’t find Earthlings very interesting, Impey suggests. “If we went to an Earth-like planet around another sun and saw some algae, we’d say ‘Well, there is algae,’ and move on,” he says. “Maybe [aliens] looked at the Earth and said the same thing: ‘Not very interesting, let’s move on.’” On the other hand, writes Shostak, maybe E.T. just doesn’t know about Earthlings. “The nearest aliens might be a few hundred or a thousand light-years distant,” he writes. “They haven't heard Ira Flatow or Lucille Ball yet.”

It’s also possible that our type of technological civilization is actually extremely rare, according to Impey. Perhaps the universe is good at making microbes, pond scum and bugs, he says, but maybe not very good at producing intelligent creations capable of communicating with us.

For his part, Impey contends that there is life out there, but when we find it, it will be microbial, and indirectly detected on an Earth-like exoplanet. As for intelligent life, he guesses that it’s either very rare or unrecognizable. SETI’s Shostak, meanwhile, is also optimistic that we’ll make contact: “I'll bet anyone a cup of Starbucks that we'll find it in the next two decades.”